1. Field of the Invention
The present invention relates to verification of digital documents.
2. Description of the Related Art
A document management system stores digital data created by a word processor or the like and those obtained by scanning documents printed on paper sheets (these digital data will be referred to as “digital documents” hereinafter). Then, the system adds electronic signatures and digital timestamps to these digital documents to implement certification of creators and the like, confirmation of non-alteration, and existence proof of the digital documents.
Note that all digital timestamps will also be referred to as electronic signatures hereinafter since they are a kind of electronic signature. Therefore, “add an electronic signature” includes cases of adding an electronic signature, adding a digital timestamp, and adding both an electronic signature and digital timestamp. Also, certification of creators and the like, confirmation of non-alteration, and existence proof of digital documents will all be referred to as “verification” hereinafter. Also, electronic signatures include digital signatures.
Under such circumstance, the application of a technique for executing batch verification of electronic signatures of a plurality of digital documents added with electronic signatures has begun. Since digital documents added with electronic signatures are increasing, a document management system having a function of executing batch verification of electronic signatures of large quantities of digital documents is in demand.
Verification of electronic signatures is a process that requires a relatively long time. In particular, a digital document, the term of an electronic signature of which has been extended many times, includes a plurality of electronic signatures. In such case, a time proportional to the number of electronic signatures is required until the verification result of that digital document is obtained. On the other hand, the technique that executes batch verification of digital documents executes identical verification processes even for digital documents which are not so important. Therefore, if there are large quantities of digital documents including a large number of electronic signatures, a long verification time is required.
Also, a document verification scheduler, which periodically verifies electronic signatures addedadded to digital documents and stores the verification results in a database in accordance with a schedule set by the user, has been proposed. With the document verification scheduler, the user determines the importance levels of digital documents to be verified, and inputs the determination results to the document verification scheduler. The scheduler verifies digital documents with lower importance levels infrequently, and verifies digital documents with higher importance levels frequently.
However, the document verification scheduler merely executes verification according to the schedule, and the verification results stored in the database by the document verification scheduler do not strictly indicate the validity of digital documents at the time of confirmation of the verification results by the user. Put differently, the document verification scheduler cannot cope with a situation in which the user wants to verify the validity of digital documents at the present moment.